Environmental control systems for aircraft typically employ air cycle machines and heat exchangers to cool and condition high pressure air supplied by either the engines or the auxiliary power unit. A compressor and fan in these machines are powered by a shaft connected to a turbine. The pressurized supply air passes first into the compressor. Outlet flow from the compressor, heated and further pressurized by the compression step, is chilled as it passes through the warm path of a heat exchanger. To sufficiently reduce the temperature of the air passing through the warm path, the fan draws cooler ambient air through the cooling path of the heat exchanger. Chilled air exiting the warm path of the heat exchanger is then expanded in the turbine to further cool it before it enters the aircraft cabin. Since the cabin air is maintained at a lower pressure than the supply air, properly designed systems provide conditioned air at temperatures low enough to cool both the cabin and the aircraft avionics.
To support the shaft connecting the turbine to the compressor and the fan, air cycle machines typically use three bearings. Two of these three bearings are journal bearings, and are configured to prevent the shaft from shifting radially. The third, a thrust bearing, fixes the axial orientation of the shaft. For optimum machine performance, very small clearances between the stators fixed to the machine housing and the tips of the fan and compressor blades must be maintained. Since the compressor and turbine rotors, to which the blades attach, are connected to the shaft, should the bearings allow more than slight amounts of free play, the shaft would shift when loaded and the blade tips would contact the stator surfaces encircling them.
As they offer minimal free play and reliable operation at high speed, hydrodynamic fluid film journal and thrust bearings are used to locate the shaft radially and axially, respectively. The inner race of each of these bearings connects to, or is a part of, the shaft, and the outer race of each attaches to the housing. When the shaft rotates, hydrodynamic forces are generated in fluid contained in the space between the inner and outer races of each bearing. These forces combine to yield a high pressure region in each bearing sufficient to oppose loads applied to the shaft.
To ensure that the magnitude of these hydrodynamic bearing forces remains constant during operation, the clearance between the inner and outer races must be maintained within a fairly narrow range. However, the hydrodynamic effect responsible for producing the high pressure region between the races of a rotating hydrodynamic bearing also generates heat. To minimize nonuniform thermal expansion and regulate inner race-outer race clearance, coolant is used to carry this heat away from the bearings.
U.S. Pat. No. 4,500,143 describes a roller bearing and journal assembly that employs oil and air both to regulate the clearance between inner and outer races and to lubricate the system. Cool pressurized oil circulates through passages adjacent to both the inner race of the roller bearing and the journal encircling the bearing. The flow rate of the cooling oil is selected to limit, during hot operation, the thermal expansion of the inner and outer surfaces to within a specified range. Holes drilled radially into the cooling passages at periodic intervals bleed a portion of this oil flow into the bearing chamber, directly lubricating and cooling the rollers comprising the bearing. To prevent the inner race from being overcooled to the point where the roller bearing and journal clearance increases beyond the specified range, warm air is introduced into a second passageway adjacent to the inner race. By applying air in this fashion, only the inner race expands, and the bearing-journal clearance remains sufficiently small.
In U.S. Pat. Nos. 4,503,683 and 4,507,939 a shaft supporting a turbine, compressor, and fan in an air cycle machine is axially and radially constrained by one air thrust, and and two air journal, bearings. A portion of the turbine inlet air is extracted, serving as a coolant that lubricates, cools, and supports these three bearings. A first portion of this coolant flows first into the thrust bearing cooling flowpath. A labyrinth seal at one end of the thrust bearing forces the coolant to exhaust from the other end. The slightly warmed coolant then flows directly into the inlet of the first journal bearing cooling flowpath. A labyrinth seal at the outlet of this cooling flowpath meters the mass flow rate of air passing through both the thrust and the journal bearing cooling flowpaths. This seal is critical, as flow exiting the first journal bearing cooling flowpath exhausts directly into the fan circuit. Without this facility for metering the cooling circuit flow, an excessive mass of air is extracted from the turbine inlet and wasted. Additionally, with no seal, the pressure of the coolant in both bearing flowpaths drops to the air pressure in the fan circuit, which is approximately equivalent to ambient pressure. As the density of the coolant at ambient pressure is too low to adequately support the bearings, the inner race contacts the outer race, causing excessive friction and potentially damaging wear.
A second portion of the coolant extracted from the inlet of the turbine is delivered to the second journal bearing cooling flowpath. The second journal bearing has labyrinth seals at both ends. The first of these seals allows no flow, and the second seal meters the amount of coolant allowed to flow through this second journal bearing, similar to the way the seal on the first journal bearing meters flow through the thrust and first journal bearings. The inlet to the second journal bearing flowpath is located adjacent to the first seal. Coolant therefore flows along the length of the bearing, exhausting through the second seal into the fan circuit.
Other, less relevant patents that generally relate to hydrodynamic bearing applications are U.S. Pat. Nos. 4,306,755 and 4,580,406.